Fast response heater for a glue gun

ABSTRACT

A heating system for a hand-held glue gun has a low-mass melting chamber, high-power resistance heating element, and thermostat ( 24 ). The heating element is arranged between the chamber and thermostat ( 24 ), so that the thermostat&#39;s ( 24 ) response to the heating element mirrors that of the load in the chamber.

CROSS REFERENCE TO RELATED APPLICATION

This application is a §371 of PCT/US99/27923 filed on Dec. 9, 1999,which was published in English as WO/00/34179 and claims the benefit ofU.S. Provisional Application No. 60/111,545 filed on Dec. 9, 1998.

TECHNICAL FIELD

This invention relates to the art of electric heating systems. Inpreferred embodiments, the invention relates to an improved electricheating element for a hand-held, hot-melt glue gun.

BACKGROUND ART

Hand-held, hot-melt glue guns are provided with a chamber for heatingand melting a solid, hot-melt material, which is typically in the formof a cylindrical stick. The material may be in any of several othershapes, including sticks having other cross sections, slugs, or chips.After melting, the composition is applied to a variety of surfaces.

The melting chamber is usually made of aluminum and has a cylindricalcavity for receiving the glue stick. The glue stick melts as it comesinto thermal contact with the interior walls of the chamber, the heatbeing transferred primarily by conduction. A cartridge heater, a PTC(positive temperature coefficient) pellet, a rope heater or a resistancewire heater is typically provided to heat the chamber, and temperaturecontrol to prevent overheating is typically provided by a thermostat.One problem with the known temperature control, however, is that ofovershoot, which arises from the fact that the chamber has thermal mass.Because of thermal inertia, the temperature of parts of the chamber,particularly those remote from the thermostat, will continue to increaseeven after the thermostat has terminated flow of electricity to theheater. The magnitude of the thermal inertia is a function of severalfactors, and the mass of the heating system is a primary factor.

Glue guns that are less expensive utilize PTC heaters because that typeof heater is cost effective. One drawback is that PTC heating elementsare of limited power, which can require the use of more than one elementwith consequent increase in cost. More power can be obtained by using aresistance heater with a discrete thermostat for temperature control,but the increased cost allows this in only more expensive systems.Further, the heat-up and melting performance of the known resistanceheater systems is only marginally better than that of the PTC systems.

In prior systems, the chamber must be large enough to accommodate theheater cartridge and the thermostat. Thus, the prior systems havesignificant physical mass, resulting in significant thermal mass andthermal inertia. This mass results in a long warm-up time, for example,of 3-5 minutes, and increased overshooting. The problems created by thisovershooting often outweigh the benefits obtained by the use of a PTCheater in the first place.

The usual heater is also provided with a silicone sleeve at the entranceto the heating chamber to provide a transition zone for the glue stick.The heating element must also heat the sleeve, by contact with theheating chamber, so that the glue stick will be able to “breakaway” andenter the heating chamber. The required heating of the sleeve increasesthe warm-up time considerably.

Thus, there is a need for a heater that is capable of heating quickly,e.g., within 30 seconds, and effectively controlling temperature tosignificantly improve the convenience of glue guns.

SUMMARY OF THE INVENTION

In accordance with the invention, a heating chamber for a glue gun iscapable of attaining an operating temperature in about 30 seconds.Further, the new heating chamber can be manufactured less expensivelythan conventional heaters and has a wider range of applications.

In a preferred embodiment of the invention, the heating chambercomprises a cylindrical tube that supports a coil of electrical heatingwire. The cylindrical tube is preferably made of metal, such asaluminum, and provides a hollow core forming a cavity for receiving theglue to be melted. The tube may be of other materials, such as ceramic,high-temperature plastic, carbon-filled plastic, and the like. The coretapers slightly longitudinally (e.g., 2°) to provide draft andfacilitate manufacture and may be die-cast or made by a screw machine.An important aspect of the inventive construction is that the thicknessof the wall between the core and the exterior is preferably no greaterthan about 0.06 inches to reduce the mass of the heater markedly. Thepreferred chamber weighs approximately 17 grams, compared with about 45grams for prior-art chambers of similar capabilities and having similarfunctions. One end of the chamber is designed to receive the glue stick,while the other is designed to receive a discharge nozzle. The nozzlemay be an integral part of the chamber, but in the preferred embodimentthe end of the chamber is internally or externally threaded forreceiving a separate, threaded nozzle with a check valve to controldripping.

The outer surface of the tube is covered with a thermally conductive andelectrically insulating film, such as a processed mica film. Other filmsmay be used, such as a polyimide film sold under the trademark Kapton, athin wall silicone film, a ceramic element or coating, a hardcoatanodized coating, and the like. As well, epoxy paint can be used as anelectrical insulator. In the preferred embodiment, the tube is wrappedtightly with the film to ensure thermal contact, and the film is thensecured to the tube by longitudinally spaced coils of 0.020 inchstainless steel lockwire. Then, a resistance heating wire, preferablymade of 80% nickel and 20% chromium and sold under the trademarkNichrome, is wrapped over the electrically insulating film to form atight coil extending longitudinally along the cylindrical tube. Theindividual coils are preferably spaced evenly along the major part ofthe tube, but it may be desired to vary the spacing to provide differentrates of heating along the chamber. For example, it may be desired toprovide more heat to the output end of the chamber. The size and lengthof the heating wire will vary depending on the specific application,including the heat to be generated and the physical dimensions of thetube.

While resistance wire is used in the preferred embodiment, it will beappreciated that other high power heating elements can be employed, suchas a deposited carbon film, a woven resistance film, a Nichrome wire ina metal sheath, and the like.

The wire in the embodiment shown may be wound onto the tube in a varietyof ways, such as by machine winding on a fixture resembling a lathe.Each end of the heating wire is secured to the tube, as by a lockwirealso. Insulated lead wires are then attached to the ends of the heatingwire, as by crimping. The coil of heating wire is then covered with asecond layer of electrically insulating film, preferably the processedmica film, and secured as with lockwire in a manner similar to that ofthe first film.

Temperature control is provided by controlling power to the heating wirewith a thermostat. This thermostat is physically attached to theinsulated surface by a mechanical fastener, such as lockwire, andelectrically connected in series with the heating coil to provide basictemperature control. By this arrangement, the thermostat picks up heatfrom the heating coil directly, which indicates the temperature of thechamber more quickly than do prior sensors that measure the temperatureof the casting directly by being embedded in the casting. Thisconstruction reduces the time necessary to respond to the heat cycle toa few seconds.

Applicant has found that this arrangement greatly reduces overshootproblems and increases the ability of the thermostat to respond to theheating requirements. For example, cold glue presents a substantialthermal load that will absorb much of the heat provided by the coil. Inthis case, absorption of heat from the coil will keep it cooler, and thethermostat will heat more slowly. As the glue heats, it will absorb lessheat from the coil, and the temperature of the coil will increase. Thus,as the glue heats up the thermostat will heat more quickly. In eitherinstance, the rate at which the heating coil heats the thermostat ismore closely tied to the rate at which it heats the load than in systemswhere the thermostat is located in the metal casting.

A basic concept of the invention is to provide a low mass, high power,fast-heating chamber controlled by a thermostat that measures thetemperature of the load indirectly by reacting to the temperature of theheating element without significant overshoot. The thermalcharacteristics of the thermostat are designed such that the temperaturedifference between the heating coil and the melted glue is the same, oressentially the same, as the temperature difference between the heatingcoil and the thermostatic element held in the thermostat's casing, whenthe glue is at the desired temperature. Thus, the mass of thethermostatic element and its casing, and the casing's area of contactwith the coil should be such that the thermostat reacts to the heatingcoil in a manner similar to that of the cylinder and the heated glue.This is particularly useful in the environment of the present inventionbecause the heating coil is of such high power compared to the mass ofthe cylindrical tube that any lag in detecting the temperature of thetube can result in melting the tube.

Other types of temperature control can be used. For example, athermistor placed in a metal casing and designed to absorb heat as setforth above could be used. In this example, a negative temperaturecoefficient (NTC) thermistor, whose resistance decreases with increasingtemperature, senses the temperature of the system, and a solid statecomparator circuit switches a relay or Triac on or off to control theheating element.

Applicant has determined that a preferred circuit for providing power tothe resistance-heating element includes a ½ wave rectifier to reduce thevoltage and resultant power delivered to the heater. This is preferredbecause it has been found more difficult to size the heating wire forglue guns that operate on 120 VAC or higher voltage. Use of a rectifiercircuit, allows the heater wire to be of heavier gauge and, therefore,more durable and easy to handle, even in small wattage ranges. Onereason the heavier wire is more durable is that it is subjected to lessthermal stress.

In a preferred embodiment, 33 Ohms of #30 Gauge wire (about 6.4 Ohms perfoot) was used to deliver about 190 Watts of power to a cylindrical tubeweighing 17 grams. This arrangement provided 10-11 Watts per gram. Aconventional arrangement provides about 40 Watts for a heater weighingabout 45 grams, or a ratio of about 1 watt per gram. Thus, a system inaccordance with the invention heats about ten times faster than theconventional system.

The preferred embodiment described has been found to heat to operatingtemperature consistently in less than 30 seconds and to provide largercontinuous melting than conventional systems. Thus, warm up time hasbeen reduced from 3 minutes to 30 seconds. Further, the melt rate wasnot compromised because the system is more responsive to temperaturedrops during normal usage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a heating chamber in accordance with theinvention.

FIG. 2 is a side view of a heating system in accordance with theinvention during assembly with a second embodiment of a heating chamber.

FIG. 3 is a side view of a completed heating system in accordance withthe invention.

FIG. 4 is a circuit diagram of a preferred control circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to FIG. 1, a preferred embodiment of a heating chamber inaccordance with the invention includes a cylindrical tube 2, which isdie cast of aluminum and includes a flange 4 for engaging a glue gunhousing (not illustrated). The tube includes a hollow core 6 with aslight draft for facilitating manufacture. The hollow core connects withan externally threaded body, which is shown as a male part 8 in FIG. 1,for engaging an outlet nozzle cap. The opposite end of the tube providesan inlet opening 10 for the glue stick.

FIG. 2 illustrates the heating system during construction with a secondembodiment of the cylindrical tube 2. In this embodiment one end isprovided with an internally threaded female opening 8′ for receiving anoutlet nozzle, shown in FIG. 3. As shown in FIG. 2, the tube 2 has beenwrapped with a thermally conductive, electrically insulating film 12 ofprocessed mica. The film 12 is secured to the tube by stainless steelwire locks 14. Then, the cylinder and film 12 are wrapped with a coil 16of Nichrome wire 18. The coil preferably extends the entire length ofthe cylinder, but the actual length and spacing of the coils will bedetermined by the heating requirements placed on the chamber. In someinstances it may be desirable to vary the spacing such that there aremore coils at the inlet end to provide more heat to the glue stick justas it enters the chamber.

FIG. 3 illustrates a complete heating system. Thus, the coil 16 has beenwound completely along the cylinder, and a second electricallyinsulating, thermally conductive film of processed mica has been placedover the coil. The ends of the heating wire 18 and the second film havebeen secured to the cylinder by wire locks 20, and a respectiveelectrical lead 22 has been attached to each end. The leads are swagedto the heating wire by known means.

A thermostat 24 having electrical leads 26 is held onto the outside ofthe coil, preferably via stainless steel wire locks 28, with theinsulating film between the thermostat and the coil. The preferredthermostat is a creep-type thermostat mounted in a stainless steel case.The thermostat case is, thus, placed in direct thermal contact with theheating wire in a substantially symmetrical fashion. This provides thenoted improvement because the thermostat is subjected to the sameheating as the cylindrical tube.

A silicone inlet sleeve 30 is secured to one end of the tube tocommunicate with the opening 10. The sleeve is held to the tube by aspring coil 32, as known in the art. The sleeve provides a transitionfor entry of the glue stick into the hollow cavity 6 of the heatingchamber. A nozzle cap 34 is threaded into the opening in the cylindricaltube.

FIG. 4 illustrates a preferred circuit for providing power to theheating wire 18. This circuit includes a series arrangement of thethermostat, the coil 16, and a diode 36 for reducing the power appliedto the wire. In the preferred embodiment wherein the heater is to beused on a hand held glue gun, the circuit is connected to a source of120 volt AC power.

The described construction has a wide range of applicability and may beapplied to virtually all known glue gun designs. Each gun, of course,requires application of individual parameters, but the design of theinvention provides the necessary flexibility for accommodating variouscombinations of power and mass. It is further envisioned that theNichrome wire could be replaced by insulated wire, which would eliminatethe need for one or more layers of insulating material and the need foraccurate winding of the coil. Also, the thermostat may be replaced witha solid-state comparator circuit or similar temperature control,particularly for glue guns operating with DC power.

The inventive arrangement has been found to provide a unique ability toaccommodate the various elements of a heater, including the specificheat of the material to be melted, the mass of the chamber and itsmass/wattage ratio, the physical properties of the wire and thermostats,and the electrical components. This design is not limited to hand-held,hot-melt glue guns, and the invention may find utility in industrialglue guns and other heating environments as well. Modifications withinthe scope of the appended claims will be apparent to those of skill inthe art.

I claim:
 1. A heating system comprising a melting chamber having lowthermal inertia, a heating element having one side in thermal contactwith said chamber and providing energy to said melting chamber at a rateof at least about 10 watts per gram weight of said melting chamber, anda thermostat in direct thermal contact with an opposite side of saidheating element.
 2. A heating system according to claim 1 wherein saidchamber comprises a cylindrical tube of metal forming a cavity.
 3. Aheating system according to claim 2 wherein said metal is die castaluminum.
 4. A heating system according to claim 2 wherein said meltingchamber has walls with a thickness of less than about 0.06 inch.
 5. Aheating system according to claim 2 wherein said thermostat comprises ametal casing in contact with said heating element.
 6. A heating systemaccording to claim 2 wherein said chamber is adapted to receive a stickof hot-melt glue.
 7. A heating system according to claim 1 wherein saidheating element is a coil of resistance heating wire.
 8. A heatingsystem according to claim 2 further comprising an insulating layerbetween said tube and said heating element.
 9. A heating systemaccording to claim 1 further comprising an electrical circuit forproviding electrical power to said heating element, said electricalcircuit comprising a half-wave rectifier.
 10. A heating system for ahand-held glue gun comprising a metal tube having a cavity for receivingsolid glue to be melted, an electrically insulating, thermallyconducting film mounted on an exterior surface of said tube, aresistance heating wire coil mounted on an exterior surface of said filmsuch that one side of said coil is in thermal contact with said tube andproviding energy to said melting chamber at a rate of at least about 10watts per gram weight of said melting chamber, and a thermostat mountedon an opposite side of said coil to absorb heat directly from said coil.11. A heating system according to claim 10 further comprising ahalf-wave rectifier power circuit in series with said thermostat andsaid coil.
 12. A heating system according to claim 10 wherein said tubehas a sidewall between said cavity and said exterior with a thickness ofless than about 0.06 inch.
 13. A heating system according to claim 12wherein said heating wire coil produces more than about 150 watts.
 14. Aheating system according to claim 10 wherein said thermostat is mountedin a metal casing, and the metal casing is in thermal contact with saidcoil.
 15. A heating system according to claim 10 wherein said coilprovides energy to said metal tube at the rate of about 10-11 watts pergram weight of said metal tube.